1. Field of the Invention
The present invention relates to an anisotropic etching mask used in a method for producing a semiconductor dynamic sensor for detecting an amount of acceleration, pressure or the like.
2. Related Arts
Semiconductor acceleration sensors, one of which is proposed in JP-A-6-104244, are conventionally used in an air bag system, an ABS system and the like of vehicles. FIG. 30 shows a conventional semiconductor acceleration sensor as an example. The acceleration sensor is composed of a silicon substrate 60, a weight 61 formed at the center of the silicon substrate 60, a rectangular frame 62 formed at the periphery of the silicon substrate 60, and thin beams 63, 64, 65, and 66 connecting the weight 61 to the frame 62. The beams 63, 64, 65, 66 have strain gauges 67, 68, 69 and 70 formed thereon respectively. When acceleration is applied to the acceleration sensor in a direction indicated by an arrow Z in FIG. 30, the weight 61 is displaced, resulting in strain stress of the beams 63, 64, 65, and 66. The strain gauges 67, 68, 69, and 70, detect the strain stress and output as an electric signal respectively.
In manufacturing processes for producing the acceleration sensor, the silicon substrate 60 is etched to form grooves 71 and the thin beams 63, 64, 65, and 66 by an anisotropic etching method using a potassium hydroxide (KOH) based etching solution or the like. When, the anisotropic etching method is performed on the silicon substrate 60, predetermined areas of the silicon substrate 60 are covered by a silicon nitride film or the like functioning as an etching mask. Under the silicon nitride film, however, the silicon substrate 60 is undesirably etched, and the corners are rounded off with sloped side walls. Accordingly, various malfunctions of the acceleration sensor are caused. Thus formed rounded corners are hereinafter called as corner-undercut portions.
To solve the problem, an anisotropic etching technique is proposed in "Si micro-machining advanced studies", PP. 117-118, published by Science forum Co. Ltd. in 1992. As shown in FIG. 31, this etching technique employs an etching mask having a rectangular pattern 72 with square form-compensation portions 73 as corner-undercut compensation patterns at the corners of the rectangular pattern 72.
This etching mask is effective in the case that a space between the weight 61 and the frame 62 is sufficiently wide as shown in FIG. 30, that is, that a distance L50 between the rectangular pattern 72 for the weight 61 and a frame pattern for the frame 62 is sufficiently wide as shown in FIG. 31. However, as shown in FIG. 32, when the distance L50 is narrow in a small size sensor, large corner-undercut portions 74 are formed under the etching mask as shown in FIG. 33 even if the etching mask has the square form-compensation portions 73.
When the corner-undercut portions are large, a width W between the beams 63 and 64, and between the beams 65 and 66, becomes narrow. Accordingly, the sensor catches the acceleration in a direction other than the direction of the arrow Z, giving rise to deterioration of directivity of the sensor. Further, the weight 61 becomes small, whereby sensitivity of the sensor decreases.